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This study was undertaken in an attempt to evaluate the relative importance of the common dental disorders. Caries and pyorrhea are considered the most common dental diseases, but the literature is somewhat confusing as to their comparative importance. Caries is said to be the most prevalent of all diseases (1), even more common than the common cold (2). Very few individuals escape its attack, its occurrence ranging from 75% to 95% in the civilized nations (3). Yet the fact has been emphasized that periodontal disease is extremely prevalent (4), with an estimated prevalence of 80% among adults (2). And it has been suspected as the cause of more extractions than any other disease (5). It is hoped that this study will present a clearer picture of the problem. METHOD The investigation consisted of the study of 1,167 consecutive registrants at the University of Michigan dental school clinic. About one-third of these underwent extractions. Using the diagnoses as made by staff members, the primary causes of extractions were divided into 8 groups:

In a recent note [Rosenberg et al., 1963a] an experiment was described in which trimethyl aluminum (TMA) was released from a rocket at night into the upper atmosphere. A persistent luminous trail formed, presumably the result of interaction of atmospheric oxygen atoms with TMA or its degradation products. The chemiluminescent trail provides an excellent marker for nighttime wind measurements in the upper atmosphere between 85 and 160 km [Rosenberg, 1964]. In this note, a similar experiment is reported in which TMA was released at twilight, so that the trail was sunlit but the sky background was dark. The purpose of the experiment was to ascertain the reaction mechanism for the TMA‐oxygen interaction by spectroscopy of the resonance radiation from reaction products or intermediate species. AlO bands were observed in the sunlit part of the trail, making possible the future utilization of TMA to obtain a diatomic molecular resonance spectrum from which ambient temperatures can be determined.

For the study of large‐scale mass‐transport processes (wind speed, direction, and shear, diffusion and turbulence), rocket‐released smoke trails have been used for altitudes below 80 km, and the twilight sodium trail has been used for higher altitudes [Manring et al., 1962]. However, the latter is restricted to a 20‐ to 40‐minute period at sunset and sunrise, when the cloud is sunlit and the sky background is dark. Two different types of payload for generating persistent, glowing trails under full night conditions have recently been designed and flown by this laboratory, and their effectiveness has been evaluated. Photographs of the trails can be treated by triangulation techniques to obtain mass motion parameters throughout the night.